Coaxial termination utilizing boron nitride member to enhance radiation



Jan. 3, 1967 Q. MA] 2; 3,296,559

R. COAXIAL TERMINATION UTIL NG BORON NITRIDE MEMBER TO ENHANCE RADI ON Filed Oct. 21, 196

INVENTOR ROBERT Q. MAINES U ited t w Patent e:

3296,59 Patented Jan. 3, 196 7 ware Filed Oct. 21,1964, Ser. No. 405,363

' 7 Claims. I (Cl. 333-22) This invention pertains to coaxial transmission line terminations in general and in particular to a reflectionless coaxial termination of the disc resistor type.

" Although conventional resistor terminations for coaxial transmission lines are well known in the art, their utilization is generally limited to low power applications (even where radial fins have been added). As the rated power of the resistive load increases, means must be supplied for expediting heat transfer, since the resistor terminates the radio'frequency energy, and as a result, its temperature builds. As a consequence, resort is generally had to water cooling and other techniques, all of which add considerably to weight and cost and impair simplicity.

Accordingly, it is the object of this invention to increase the power rating'of resistive (particularly disc type) terminations and to do so simply and economically with negligible increase in weight.

The above mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will best be understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a side section of one embodiment of the termination of the invention; and I FIG. 2 is a section along line 2-2 of FIG. 1.

Turning now to the figures, there may be seen a circular multi-element body, the left end (FIG. 1) of which is adapted for a type-N connector (although any type may be used). In this type connection, a spring fingered bullet (not shown) is inserted between the terminating center conductor 10 and the corresponding line center conductor (not shown). The outer conductor of the line is then positioned by means of a hub (not shown) which enters the annular recess 12 of the terminating outer conductor, and is rigidly secured in place by means of bolts passing through the first radial fin 13 and a corresponding flange in the connecting line.

The circular resistive disc 20 is of the conventional type, well known in the art, and comprises a thin resistive film (for example, of nickel chrome) vacuum deposited upon a suitable dielectric substrate (such as beryllium oxide, aluminum oxide, Pyrex, etc.). When necessary, a further film is added to prevent the resistor from oxidizing. Annular tin rings (not shown) are provided at the inner and outer diameters for accepting good ohmic connections to the resistor. The disc resistance naturally depends upon the thickness of the film which is predetermined to give the desired terminating resistance; i.e. the characteristic parameter.

At this juncture, it bears mentioning that in the conventional termination, the disc is mounted electrically and mechanically between the inner and outer conductors with the resistive film facing inward (in the direction which RF energy is being received). So disposed. the inner conductor may then be directly connected to the resistive film. As a consequence, the heat must be radially conducted off the film to contiguous fins.

ing plate 23.

The disc of the invention is mounted backwards, however, that is, with the substrate, as opposed to the film (shown by the heavy line), facing inward. Electrical contact with the inner conductor isimparted by the screw 17, the head of Which is soldered to the tinned portion of the disc, and the end of which is embraced by a tapped hole in the inner conductor 10. The tinned outer periphery of the resistivedisc is soldered to an annular flange 19, the electrical purpose of which will be explained.

The disc and annular flange are disposed within a cup shaped cavity formed in the radiating member 21; the open end of the cavity being closed by the radiating and retaining member 23. Within the closed cavity and contiguous most of its exposed surface is a boron nitride cylinder 25.

Having described the mechanical configuration of the termination, its thermal and electrical characteristics will now be analyzed.

While as a general proposition, thermal and electrical conductivity are analogous in most materials, boron nitride is an exception. That is, while it is a poor conductor of electrical energy, and thus has the properties of a dielectric, it is an excellent thermal conductor, as well as having refractory characteristics; its upper temperature limit far exceeding that which would degrade dielectrics such as Teflon. It may therefore abut directly upon the resistive film without impairing its (the films) function. Thus, instead of merely conducting the heat from the resistive film radially, as is the conventional cooling method, the heat is conducted axially through the boron nitride and then radially and axially to the spaced, preferably black, radiating fins 13 to 16, and terminat- Needless to say, heat dissipation is greatly enhanced and employing the inventive arrangement, a resistive disc rated at one watt was found to withstand a 2000% power increase without degradation.

Electrically, as is well known, a resistive disc is not a true ideal termination because of the impedance discontinuity at its boundary. To avoid reflections, it must be matched out. This is accomplished in the described embodiment as will be described hereinafter. Since the dielectric material between the inner conductor 10 and outer conductor 21 is air, the characteristic impedance, a function of dielectric constant, would invariably change upon encountering the disc substrate. To compensate for the increase in dielectric constant, the ratio of diame-ters must be correspondingly increased in accordance with the expression:

where D and d are respectively the inner diameter of the outer conductor and the outer diameter of the inner conductor. As may be seen in FIG. 1, the characteristic impedance in the substrate region depends upon the screw and cavity diameters. This gives a ratio greater than that in the preceding region, and satisfies the foregoing equation.

A further consideration is the shunt capacitance offered by the disc. This is compensated by the series inductance of a short segment of higher characteristic impedance transmission line, just to the left of the disc. The parametric value of the section is determined by the inner flange diameter of the annular member 19 and the outer diameter of the inner conductor 10.

Finally, the region beyond the resistive disc must appear as an open circuit to the RF energy. This, however, presents little problem in the described embodiment, since with the screw head maintained as thin as possible, the remaining region, embraced by the boron nitride, represents a conversion from coaxial line to circular waveguide (i.e. TEM to TE propagation modes) which operates below cutoff.

An example of the above disc embodiment for a 50 ohm characteristic termination follows:

Inches Coaxial Inner Conductor Diameter (O.D.) .120 Coaxial Outer Conductor Diameter (I.D.) .276 Screw Diameter .072 Cavity Diameter .440 Inner Diameter of Inner Flange Member .330 Thickness of BeO Disc .062 Thickness of Inductive Region .050 Length of Cavity .300 Overall Termination Dimensions 1.5 .5

The foregoing termination not only greatly improved the capacity of the employed resistive disc, but it gave the following excellent SWR characteristics:

Frequency: SWR D.C.l.OKmc 1.02 l.0-3.0K mc. 1.07 3.0-4.0K mc. 1.10 4.0-6.0K mc 1.25

Although the boron nitride has been shown and described as cylindrical in form, it may be easily reduced in size to particles of the order of microns and the cavity potted. Further, the boron nitride may be employed in the conventional arrangement (the resistive film disposed inwardly). This will, however, introduce additional and not easily resolvable factors in matching out the termination and is therefore not the preferred arrangement. Since boron nitride has a tendency to oxidize, an agent such as silicon may be added to retard oxidation.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. A termination for coaxial transmission line comprising: a resistor; means for coupling said resistor be- .4 tween the inner and outer coaxial conductors; a radiating member disposed adjacent said resistor; and boron nitride in intimate contact with said resistor and said radiating member for conducting heat therebetween.

2. The termination claimed in claim 1, in which said resistor is disc-shaped.

3. A termination for coaxial transmission line comprising: a cylindrical radiating member having a cavity therein; a disc-shaped resistor mounted in said cavity and peripherally coupled to said radiating member; means for coupling RF energy to the resistive disc; and boron nitride within said cavity in intimate contact with the resistive portion of said disc and the cavity periphery for conducting heat between said resistive disc and said radiating member.

4. The termination claimed in claim 3 in which said cavity is cylindrical; in which said boron nitride is cylindrical in form; and in which said radiating member includes a portion closing said cavity substantially parallel to said resistive disc, the end faces of said boron nitride cylinder being in respective intimate contact with said closing portion of said radiating member and said resistive disc.

5. A termination for coaxial transmission line comprising: a cylindrical radiating member having at least two axially communicating cylindrical cavities therethrough; a disc-shaped resistor mounted in one of said cavities and peripherally coupled to said radiating member; means closing the cavity containing the resistive disc substantially parallel thereto; boron nitride disposed in said one cavity in intimate contact with said closing means, the resistive portion of said disc, and the cavity periphery; and means including the other of said cavities for coupling RF energy to the resistive disc.

6. The termination claimed in claim 5, in which said termination comprises means for matching the resistive disc to the predetermined characteristic impedance of the termination.

7. The termination claimed in claim 5, in which the RF energy coupling means comprises a dead ended inner conductor; and means for connecting said conductor to the resistive portion of said disc.

No references cited.

HERMAN KARL SAALBACH, Primary Examiner. R. F. HUNT, JR., Assistant Examiner. 

1. A TERMINATION FOR COAXIAL TRANSMISSION LINE COMPRISING: A RESISTOR; MEANS FOR COUPLING SAID RESISTOR BETWEEN THE INNER AND OUTER COAXIAL CONDUCTORS; A RADIATING MEMBER DISPOSED ADJACENT SAID RESISTOR; AND BORON NITRIDE 